The final installment of Blackadder, Back & Forth is set on New Year's Eve in 1999. The modern day Lord Edmund Blackadder and his idiotic servant Baldrick construct a time machine designed by Leonardo da Vinci as part of a prank on his party guests, betting them to give £10,000 each if he brings back a nominated item of historic significance for him to bring back - namely a Roman centurion helmet, the Duke of Wellington's iconic wellies, and a stinking pair of 18th Century underpants (provided by Baldrick). To their surprise, the time machine actually works, landing in the Cretaceous Period where they are attacked by a Tyrannosaurus Rex. Baldrick tosses the collected props at the dinosaur, successfully killing it with his underwear (apparently wiping out all dinosaurs with the stench).
Blackadder and Baldrick try to return home, but Baldrick never got around to writing in dates to know where they are going. By chance, the machine lands back in Blackadder's manor, but during Elizabeth I's reign. Blackadder manages to escape execution by giving the queen some Polo mints, and she gives him her crown as a prize. Heading back to the machine, Blackadder bumps into William Shakespeare. He gets Shakespeare to give him an autograph using a ball point pen, then punches the Bard, blaming him for misery his plays will bring to children in the future. Blackadder returns to the time machine, though forgets to take the pen back from Shakespeare.
After a brief encounter with The Future, Blackadder and Baldrick end up in Sherwood Forest and meet Robin Hood (evidently an ancestor of Lord Flashheart), his merry men and Maid Marian. However, Blackadder questions the pointlessness of their good deeds, prompting the men to kill Robin by shooting him with arrows, following which he gets off with Maid Marian. Later, the time machine crash-lands on The Duke of Wellington, killing him just before the Battle of Waterloo, though Blackadder is able to steal his boots before things kick off. The time machine then lands by Hadrian's Wall in Roman Britain. Blackadder and Baldrick spot their Roman counterparts, who learn from General Melchecus that they are to return to Rome. A horde of Scottish warriors charge the wall, prompting Blackadder and Baldrick to flee in the time machine.
Baldrick comes up with a cunning plan to remember how the machine's levers were set before they started, which involves Blackadder almost drowning him in the toilet. It works, and they return to the present only to discover history has been drastically changed — no one's heard of Robin Hood, Shakespeare is only known as the inventor of the ball-point pen, and, to Blackadder's utter shock, the French rule Britain thanks to Napoleon Bonaparte's victory at Waterloo. Blackadder and Baldrick hastily go back into the past to make amends, and Blackadder wins his bets. Melchett comments on how history could be changed by an unscrupulous person, giving Blackadder an idea, and he goes on a final trip in the time machine.
Blackadder's friends go upstairs to watch the countdown for the Turn of the Millennium on television. On TV, a royal car rolls up and out steps Blackadder — now King Edmund III — accompanied by Maid Marian as his queen. Baldrick has become Prime Minister, and maintains a strong friendship with Blackadder. The end credits end with a teaser claiming that the sequel will be released in the year 3000!
- Achievements in Ignorance: Baldrick accidentally creates a working time machine.
- Actor Allusion: In the opening credits, there is a photo of Blackadder in Indian attire. This is a homage to Rowan Atkinson's then-wife Sunetra Sastry, who was makeup on the second season and is of Indian descent.
- And This Is for...: Blackadder punches William Shakespeare and says "that is for every schoolboy and schoolgirl for the next four hundred years".
- Aside Glance: The final shot has Edmund and Baldrick glancing to the audience, Blackadder raising a Fascinating Eyebrow and Baldrick winking.
- Bestiality Is Depraved: Blackadder makes a crack during the Roman period about Baldrick having sexual relations with a sheep.
- Book Ends: The first Blackadder series was all about Prince Edmund wanting to become king. In this special, the last of the Blackadder series, his descendant finally achieves it note .
- Bungling Inventor: Good news — Baldrick's time machine works. Bad news — He used a slot machine for the year readout and forgot to put numbers over the fruit symbols.
- Camp Gay: Robin Hood calls Will Scarlett "a poof in tights", and the camera cuts to him waving flirtatiously at Blackadder and Baldrick. It's later implied that Baldrick had a romantic or at least very friendly encounter with him, which Blackadder is too stunned by to pass comment on.
- Canis Latinicus: Melchett's Roman ancestor speaks perfect Latin, but ends his report with "Beeeh-us!"
- Casting Gag: Colin Firth's cameo as Shakespeare was likely an allusion to his role as Lord Wessex in Shakespeare in Love.
- Cheese-Eating Surrender Monkeys: Inverted. After Blackadder accidentally kills the Duke of Wellington, Napoleon wins the Battle of Waterloo and succeeds in conquering Britain, rendering it (and presumably the rest of the British Empire) subservient to France. He discovers this when he returns home and is offered his reward in francs and told that the others are about to watch the President of France's New Year's speech, upon which they start singing La Marseillaise. Blackadder is horrified by this.
- Continuity Nod: Just before they set off on their final trip through time Baldrick asks how cunning Blackadder's plan is by comparing it to a fox who used to be Professor of Cunning at Oxford University, a simile Captain Blackadder used when asking about Baldrick's last plan to get out of the "Big Push" in "Goodbyeee" (while adding the detail that said fox has now moved on to work for the United Nations' Department of International Cunning Planning).
- Even Evil Has Standards: Blackadder may be a scoundrel, but he's a British scoundrel. The realization that he'd inadvertently caused England to be conquered by France horrifies him into go back into the time machine to undo the damage he'd done to history.
- Expy: By the admission of both Rowan Atkinson and the writers, the present-day Lord Blackadder seen here is effectively the Blackadder II incarnation transplanted to the 20th/21st century. Also, his time machine is more or less one for the TARDIS.
- Giving Radio to the Romans: Because he forgot to take his pen back when getting Shakespeare's autograph, Blackadder created a new timeline where the ballpoint pen was invented a few centuries earlier.
- Good Is Dumb: How Blackadder convinces the Merry Men to turn on Robin Hood. However, he quickly realises what a big mistake that was.
- Grand Finale: Unless there really is another Blackadder entry planned for release in the year 3000.
- Historical In-Joke: In this version of history, the dinosaurs were killed by the smell of Baldrick's underwear. (And unlike every other change Blackadder and Baldrick make to history, they never undo this one.)
- Identical Grandson: Everyone! In fact, the only characters who don't have an identical ancestor or descendant somewhere else in the Blackadder series are William Shakespeare and Maid Marian.
- Lighter and Softer: Though it still has the series' trademark wit and Black Comedy, the vulgarity is toned down noticeably. Blackadder, while still a jerkass, is more a petty schemer that gets caught in a plan gone wrong, and perhaps most noticeably the film ends on a surprisingly happy note that isn't at the expense of any of the main characters.
- My God, What Have I Done?: Blackadder's reaction when he discovers that he's altered history and caused Britain to become a French colony. It's implied that he could have lived with Robin Hood and the works of Shakespeare being erased from history, but not this — although he ends up restoring everything. For a few minutes, at least.
- Naked Apron: How Baldrick serves dinner at the beginning of the episode. Definitely a Fan Disservice example.
- Newscaster Cameo: The voice of the Royal Reporter in the ending scene was Jennie Bond, The BBC Royal correspondent from 1985 to 2003.
- Noodle Incident: Baldrick's doing the cooking because Lord Blackadder's regular chef has been invited to "an orgy at Delia Smith's".
- Oddball in the Series: The only installment of Blackadder without a laugh track.
- Offscreen Moment of Awesome: We never see precisely how Blackadder changes history to become king, presumably it involved changing the ending of the first season so that the first Blackadder wasn't tortured or poisoned to death.
- Product Placement: Tesco was among the sponsors for the special, hence the scene where Blackadder tries to offer his loyalty card to Queenie.
- Richard Nixon, the Used Car Salesman: When Blackadder accidentally left his pen with William Shakespeare, a new timeline was created where Shakespeare never wrote any plays, but did invent the ballpoint pen.
- Shout-Out: The time machine falls on the Duke of Wellington in a similar manner to Dorothy's house in The Wizard of Oz.
- Take That!: Blackadder disparages Shakespeare for giving them Ken Branaghs endless, uncut, 4-hour version of Hamlet.
- Throw the Dog a Bone: The episode ends with Blackadder dynasty finally achieving greatness, as Edmund uses the time machine to ensure that he becomes King. It works the Baldrick family too, as one of them gets to be Prime Minister (a puppet PM given that King Edmund has abolished Parliament, but still).